I know that images of 950 pixels wide (or above) are selected for the slideshow, and as such I specifically photoshop my blog images to be either 900 or 950 depending on which ones I want to be shown in the slideshow. It has always reliably shown the images that I have wanted, based on this practice, and cropped the height of the image to fit the slideshow size. I quite like this cropping, as it gives a sort of "detail" version of the images. But when I made my most recent post and included an image of 950px wide, instead of cropping, it has left the length, but resized the height of the images to fit the slideshow, giving a distorted appearance.